John, a couple of thoughts.
Unless you have a central vac that is ducted to the outside, a standard vacuum just tends to spread things around. At the last house, I had a steam cleaning company do my concrete floor before I had the carpet laid. They looked at me funny - but to me, it made a huge difference.
A HEPA filter will help out with air bourne dust, and is great for particulate that you
can't see. Most of the dust you see on your furniture and electronics was heavy enough to fall out of the air before it got anywhere near the filter.
The portable units are great for small enclosed (sealed) rooms, but not for entire basements. Also, because of its smaller dimensions the portables tend to load very quickly, requiring frequent filter changes. If the unit was inexpensive (read cheap), the motor won't be able to handle the added resistance when the filter is dirty, and burn out quickly. The cheap ones also tend to have crappy seals and suffer from "blow by". (100% of the air isn't going through the filter)
As a side note, I'd ear mark a higher percentage of funds for sound proofing that furnace room located RIGHT beside your seating.
BTW - What's the story on the vault?
